European Patent No. 0 558 922 describes a method for improving motion estimation in image sequences, in half-pel accuracy, according to the full-search method. There, in a first process step, the search area, and in a second process step, the match block are filtered with the aid of an additional digital filter which enables a raster shift of the pixel raster by ¼ pel. Using this measure, a distortion of the motion vector field can be ruled out.
In “MPEG-4 Video Verification Model Version 7.0”, Bristol, April 1997, MPEG 97/N1642 in ISO/IEC JTC1/SC 29/WG11, an encoder and decoder for object-based coding of video image sequences are specified. In this context, one no longer encodes and transmits rectangular pictures of a fixed size to the receiver, but instead, so-called “VIDEO OBJECTS” (VO) of any shape and size. The image formation of such a VO in the camera image plane at a specific instant is referred to as a VIDEO OBJECTS PLANE (VOP). Consequently, the relation between VO and VOP is equivalent to the relation between image sequence and image in the case of the transmission of rectangular pictures of fixed size.
The motion-compensating prediction in the verification model is carried out with the assistance of so-called “blockwise motion vectors” which, for each block of the size 8×8 or 16×16 pixels of the current image, specifies the position of the block used for the prediction in an already transmitted reference image. In this context, the resolution of the motion vectors is limited to half of a pixel, pixels between the scanning raster (half-pixel position) being generated by a bilinear interpolation filtering from the pixels on the scanning raster (integer pixel position) (FIG. 1). In this case, + indicates the integer-pixel position, and O the half-pixel position. Interpolated values a, b, c, and d in half-pixel position are derived from the following relations:a=A, b=(A+B)//2, c=(A+C)//2,d=(A+B+C+D)//4, // indicating a rounded integer division.